Prelab problem: In this experiment, you will apply a horizontal
force to a box loaded with weights, in order to pull the box at constant
velocity across a horizontal plane. Follow the usual force diagram procedure
to explain why

1) the magnitude of the force of the pull will equal the magnitude
of the force of kinetic friction, and 2) the magnitude of the weight will
equal the magnitude of the normal force.

Added Mass (kg)

Total Weight (N)

Pull Force

(N)

0.200

0.400

0.600

0.800

1.000

1) Use a 5 N spring scale to measure the weight of
the wooden box, and record your result. Then, using the spring scale, apply
a HORIZONTAL force to the box to pull the box at a constant, slow velocity
along the HORIZONTAL friction plane (NOT along the classroom table !).
Try to make the box's velocity as constant as you can - start with a gentle
pull and increase your pull gradually until you can move the box with constant
speed. Read the force of pull for five values of mass (0 to 1 kg in increments
of 0.2 kg) placed in the box. Distribute the weight in the box as uniformly
as possible. Make a table of the weight of the loaded box (box + added
weight) and the corresponding pull force.

2) As you were carrying out the measurements above,
what did you notice about the reading on the scale just as the box began
to move? Make a conclusion about the difference between the friction force
on an object at rest and that on the object while moving.

3) Use your TI calculator to plot and fit graph of
FRICTION FORCEvs NORMAL FORCE. (Where will the data come
from?) Sketch the graph in your lab journal, and write the equation of
the fit in the usual way (math equation, physics equation, matching table,
physics with fit coefficients -- numbers and units -- substituted, etc.).

4) If 0.700 kg of mass were placed in the box, what
would you expect the force of friction to be? Use your equation of fit
to calculate the answer. Then check your answer by making the measurement.
Quantitatively compare the measured value to the calculated value. Show
all work for predictions, and record all measurements in your lab journal.
Compare the prediction with measurement (% difference?).

Prelab problems:
In the experiment you just completed, the weight of the box was equal to
normal force of the table on the box, and the horizontal pull was equal
to the friction force. In the next experiment, you will incline the friction
plane. In this situation, how will the normal force be related to the weight?
the pull related to the friction force? To answer these questions, do the
following in advance of the lab.

a) On a diagram in your lab journal, draw and label
the forces acting on the box. The box is being pulled at constant velocity
upward along the inclined plane and parallel to it. Use axes parallel and
perpendicular to the plane.

b) Write the net force equation for the axis perpendicular
to the plane. Then solve for the normal force in terms of the weight of
the box, mg, and the angle of inclination, q
. Make it clear on your diagram how the angle is used in finding force
components.

c) Write the net force equation for the axis parallel
to the plane. Then solve for the friction force in terms of the force of
the pull, mg, and q
.

When your answers to the prelab problems are complete,
do the following.

1. Using a box loaded with 400 g, pull the box at
constant velocity upward along the plane for angles

of 0 to 50°
in increments of 10°
. Be sure to keep the pull parallel to the plane. Record your results

in a table similar to the one below. Use your results
from b) and c) to calculate the normal and

friction forces.

Record the weight of the loaded box.

Angle

(°)

Pull Force (N)

Normal Force (N)

Friction Force (N)

2. Using your TI calculator, fit a graph of FRICTION
FORCE vs NORMAL FORCE. Sketch the graph in your lab journal (label axes
with quantities plotted and units) and give the results of the fit in the
usual way.

3. Compare your results to those of step 3 in Part
A.

C) More about Normal and Friction Force

1) Now put the box, loaded with 400 g, on a horizontal
friction plane. Pull the box with the spring scale at constant velocity,
exerting the force at an angle of 30° to the horizontal. Give the value
of the pull force that you measured. Then calculate what the friction force
must have been. Explain your answer using a force diagram and net force
equation.

2) Given the friction force determined in step 1,
what would you expect the normal force to be? Use a result from Part A
or B in determining your answer.

3) Calculate the normal force by a second method,
using your force diagram from step 1 and another net force equation.

4) Quantitatively compare your answers to 2 and 3.

D) Coefficient of static friction

Prelab problem:
In this experiment, you will use a simple technique to measure the coefficient
of static friction. The method involves placing a box on an inclined ramp
and raising the ramp gradually until the box just begins to slide. The
angle at which this occurs is called the critical angle, q
c. Following the usual force problem procedure (using a force diagram,
net force equations, etc.), show that the maximum angle for which the box
will not slide down the ramp is q
c = tan-1(µs), where µs is the coefficient of static
friction.

1) Use the method described in the pre-lab to determine
µs for a box loaded with 400 g. How does your result compare to the
coefficient of kinetic friction obtained in parts A and B ?